The emissions of hydrochloric acid and sulfur oxides have challenged combustion plant operators and regulators since there became an awareness of the harmful effects of acid rain. These materials have recently taken on regulatory and technical momentum.
Being acid materials, the art has generally employed alkali- or alkaline earth-containing sorbents to control them. A variety of wet and dry scrubbing and sorbent injection techniques have been employed, but the scrubbing techniques typically require the installation of large and expensive capital equipment and add significant solids that need processing.
Many older plants are averse to the installation of large capital equipment owing to lack of space and uncertainty with respect to future retirement dates. Dry sorbent injection offers a lower-capital alternative to wet and dry scrubbers but adds solids that must be recovered and disposed.
Calcium-containing sorbents are often less efficient in capturing SO2 and HCl than sodium-based sorbents, and calcium-based sorbents are highly resistive. As such, large quantities of sorbent often must be used to comply with HCl and SO2 emissions limits, and the resistive nature of calcium-containing sorbents can negatively impact the operation of electrostatic precipitators used to remove particulate matter. Negatively impacting electrostatic precipitation can result in increased particulate emissions and violation of particulate emissions limits.
Injection of sodium-containing sorbents into ducts can be used to efficiently reduce HCl and SO2 with minimal impact on electrostatic precipitation. However, sodium-containing sorbents are difficult to handle (e.g., hygroscopic), can ruin the ability of fly ash to be used in the manufacture of concrete, and increase the solubility of heavy metals in fly ash ponds, and ultimately results in the leachability of heavy metals (e.g. arsenic and selenium) into the environment.
Accordingly, there is a present need for a process that can reduce HCl and/or SO2 emissions from combustion gas streams by employing a non-sorbent material. It would be especially helpful if such a composition was in the form of a molecular reactant (as opposed to a solid sorbent) which could chemically convert the chloride content of HCl to a stable chemical form that could be easily removed from the system by cloth filters, electrostatic precipitators or like solids recovery apparatus.